Matter In Our Surroundings

Quickly browse through questions and notes on related topics. You can also download and read this topic offline.

• Questions
• Summary

A surface phenomenon in which liquid changes into vapours at any temperature below its boiling point is called evaporation. Particles on the surface of a liquid have higher kinetic energy than others, so they break. The forces of attraction between the particles & escape from the surface of the liquid in the form of vapours.

Factors affecting evaporation: Rate of evaporation depends on :

1. Exposed surface area: Evaporation takes place more rapidly with the larger surface area. This means the rate of evaporation increases with increase in surface area and decreases with a decrease in surface area.
2. Pressure: Evaporation decreases with increase in pressure and increases with a decrease in pressure. This means the rate of evaporation is indirectly proportional to the pressure.
3. Humidity: Evaporation decreases with increase in humidity and increases with a decrease in humidity present in the air. This means the rate of evaporation is indirectly proportional to the humidity present in the air.
4. Wind: Evaporation increases with the increase in wind speed and decreases with a decrease in wind speed. This means the rate of evaporation is directly proportional to the speed of the wind.
5. Temperature: Evaporation increases with increase in temperature and decreases with a decrease in temperature. This means the rate of evaporation is directly proportional to the temperature.

The cooling caused by evaporation is based on the fact that when a liquid evaporates, it takes the latent heat of vaporization from surroundings which on losing heat get cooled.

Examples :

1. When we put acetone on our hand, it gets vaporized by taking heat from our hand and our hand feels cool.
2. We should wear cotton clothes in summer to keep cool and comfortable as cotton is a good absorber of water, so it absorbs the sweat from our body and exposes it to air for evaporation of sweat thus cools our body.
3. Often people sprinkle water on the ground during summer. This water takes heat from the ground and surrounding air to evaporate, thus making the place cool.